As a circuit for dividing or coupling high frequency signals, those of a type proposed by E. J. Wilkinson have been known. The circuit includes an input terminal which is grounded through a grounding capacitor, and a pair of output terminals connected respectively through coils with the input terminal. Each of the output terminals is grounded through a grounding capacitor. A balancing capacitor and a resistor are connected in parallel with each other between the pair of output terminals. The high frequency signal dividing and/or coupling device is used to divide a high frequency signal applied to the input terminal into two high frequency signals of the same level and the same phase. The divided high frequency signals are produced in the pair of output terminals. When high frequency signals are applied to the respective ones of the output terminals, it is possible to obtain at the input terminal a high frequency signal which is produced by coupling the high frequency signals applied to the output terminals. Thus, when the high frequency signal dividing and/or coupling device is used for coupling signals, the input terminal functions as an output terminal and the output terminals function as input terminals.
Conventionally, it has been known to form high frequency circuits having coils and/or capacitors from sintered materials. In the conventional process, a green sheet of a dielectric material is provided with patterns of coils and/or capacitors by printing or coating thereon a conductive material, and a plurality of such green sheets are laid sequentially one over the other to form a block which is then subjected to a sintering process under a predetermined sintering temperature to provide a desired high frequency circuit of a sintered integral structure. An example of such process for producing a high frequency circuit is disclosed by the Japanese Laid-Open Patent No. Hei 4-355902.
It should however be noted that several problems are encountered when efforts are made to form the aforementioned high frequency signal dividing and/or coupling device with the known laminated structure. For example, a plurality of layers in the laminate have to be provided with patterns of capacitor electrodes for forming three input and output capacitors and a balancing capacitor between the output terminals as well as patterns for grounding electrodes. In this structure, there may be produced unexpected capacities which may be called as floating capacities between the capacitor electrodes and the grounding electrodes. The floating capacities can cause an imbalance between the paired output grounding capacitors so that the two outputs may have unequal impedance. As a result, there may be produced a difference in the level of the two output terminals thereby making it impossible to divide the output with equal levels. It is extremely difficult to eliminate the difference in level between the output terminals simply by appropriate design of arrangements of the capacitor electrodes.
It should further be noted that when the signal dividing and/or coupling device is to be designed for use in a very high frequency range such as above 1 GHz the output grounding capacitors have to be of a very small capacitance such as less than 0.5 pF. Such small capacitance is apt to be affected by floating capacitances caused for example by wiring so that it is difficult to determine the capacitance by an appropriate design of patterns. In the case of a circuit of a laminated structure, capacitance must be adjusted between each adjacent layer, however, difficulties arise in that any deviation in the layer thickness can affect the capacitance. The balancing capacitor between the paired output terminals has an influence on the isolation between the two outputs. It should be noted that when it is desired to provide a high isolation between the outputs the frequency at the center of the isolation can easily be shifted by a deviation in the thickness of the layer.